This present invention relates to woven dual layer papermaking fabrics and more particularly to forming fabrics which are adapted for use in papermaking machines.
Dual layer papermaking fabrics are usually woven with a single set of machine direction yarns which are interlaced with and bind together two sets of cross-machine direction yarns. The cross-machine direction yarns normally are at least partially arranged in a vertically stacked manner.
Dual layer papermaking fabrics are manufactured in two basic ways, i.e. they are woven endless with there being only one set of weft yarns which extend in the machine direction or they are woven flat with there being only one set of warp yarns which extend in the machine direction. The papermaking fabric of the invention is preferably woven flat.
Papermaking fabrics must possess stability in both the machine and cross machine direction so that uniform drainage and uniform drying of the paper product occurs. The papermaking fabric must possess a smooth, tightly woven paper support surface with smooth evenly formed knuckles to minimalize wire markings and to provide for uniform marking throughout. Finally they must have extended wearability.
Efforts to maximize each of these requirements to date have not been totally successful. In order to increase wear, it is known to use coarse weft yarns for the lower layer. It is also known to weave the weft yarns so that extended weft floats appear also on the running or machine surface. Usually the warp yarns are woven with the weft so that there are minimal appearances of the warp yarn on the running surface.
In order to provide a substantially planar paper support surface with adequate drainage, smaller and in some instance a greater number of weft yarns are woven with the warp yarn to form the upper layer or support surface of the fabric. This weave must be stable so that uniform and minimal markings are made on the paper sheet while allowing for the necessary drainage through the fabric.
Problems, particularly on the running surface, remain with this construction. The use of large and stiff weft yarns woven in a pattern which products long or extended floats have in the past created uneven drainage and an unstable surface on both the support and machine surface. Because of its stiffness the weft yarns create floats which tend to loop or not lie flush against the warp yarns. These floats have a tendency to slip or sag in a manner as to become un-stacked or mis-aligned with the weft yarn of the opposite layer. This movement of the weft yarn floats creates uneven drainage cavities which results in uneven drainage. It also creates a surface in which the knuckles are not uniformly arranged which causes the markings on the paper sheet to be non-uniform.
Accordingly, it is an object of this invention to provide a multilayer papermaking fabric which overcomes the above set forth conditions;
Another object of the present invention is to provide a multilayer papermaking fabric having machine surface weft yarn floats which provide improved wear resistance and in which the weft yarn floats are securely bound with the warp yarns;
Another object of the present invention is to provide a papermaking fabric which maintains constant drainage during use;
Another object of the invention is to provide a papermaking fabric in which the support and machine surfaces are woven in a balanced twill weave pattern;
Another object of the invention is to provide a papermaking fabric in which the balanced twill weave pattern of the support surface and the machine surfaces differ;
Another object of the invention is to provide a multilayer papermaking fabric which is capable of producing fine paper.